Based on kernel version 2.6.27. Page generated on 2008-10-13 09:53 EST.

1Using the RAM disk block device with Linux
2------------------------------------------
34Contents:
56 1) Overview
7 2) Kernel Command Line Parameters
8 3) Using "rdev -r"
9 4) An Example of Creating a Compressed RAM Disk
1011121) Overview
13-----------
1415The RAM disk driver is a way to use main system memory as a block device. It
16is required for initrd, an initial filesystem used if you need to load modules
17in order to access the root filesystem (see Documentation/initrd.txt). It can
18also be used for a temporary filesystem for crypto work, since the contents
19are erased on reboot.
2021The RAM disk dynamically grows as more space is required. It does this by using
22RAM from the buffer cache. The driver marks the buffers it is using as dirty
23so that the VM subsystem does not try to reclaim them later.
2425The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
26to support an unlimited number of RAM disks (at your own risk). Just change
27the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
28and (re)build the kernel.
2930To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
31directory. RAM disks are all major number 1, and start with minor number 0
32for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
3334The new RAM disk also has the ability to load compressed RAM disk images,
35allowing one to squeeze more programs onto an average installation or
36rescue floppy disk.
3738392) Kernel Command Line Parameters
40---------------------------------
4142 ramdisk_size=N
43 ==============
4445This parameter tells the RAM disk driver to set up RAM disks of N k size. The
46default is 4096 (4 MB) (8192 (8 MB) on S390).
4748 ramdisk_blocksize=N
49 ===================
5051This parameter tells the RAM disk driver how many bytes to use per block. The
52default is 1024 (BLOCK_SIZE).
5354553) Using "rdev -r"
56------------------
5758The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
59as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
60to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
6114 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
62prompt/wait sequence is to be given before trying to read the RAM disk. Since
63the RAM disk dynamically grows as data is being written into it, a size field
64is not required. Bits 11 to 13 are not currently used and may as well be zero.
65These numbers are no magical secrets, as seen below:
6667./arch/i386/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF
68./arch/i386/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000
69./arch/i386/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000
7071Consider a typical two floppy disk setup, where you will have the
72kernel on disk one, and have already put a RAM disk image onto disk #2.
7374Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
75starts at an offset of 0 kB from the beginning of the floppy.
76The command line equivalent is: "ramdisk_start=0"
7778You want bit 14 as one, indicating that a RAM disk is to be loaded.
79The command line equivalent is: "load_ramdisk=1"
8081You want bit 15 as one, indicating that you want a prompt/keypress
82sequence so that you have a chance to switch floppy disks.
83The command line equivalent is: "prompt_ramdisk=1"
8485Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
86So to create disk one of the set, you would do:
8788 /usr/src/linux# cat arch/i386/boot/zImage > /dev/fd0
89 /usr/src/linux# rdev /dev/fd0 /dev/fd0
90 /usr/src/linux# rdev -r /dev/fd0 49152
9192If you make a boot disk that has LILO, then for the above, you would use:
93 append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
94Since the default start = 0 and the default prompt = 1, you could use:
95 append = "load_ramdisk=1"
9697984) An Example of Creating a Compressed RAM Disk
99----------------------------------------------
100101To create a RAM disk image, you will need a spare block device to
102construct it on. This can be the RAM disk device itself, or an
103unused disk partition (such as an unmounted swap partition). For this
104example, we will use the RAM disk device, "/dev/ram0".
105106Note: This technique should not be done on a machine with less than 8 MB
107of RAM. If using a spare disk partition instead of /dev/ram0, then this
108restriction does not apply.
109110a) Decide on the RAM disk size that you want. Say 2 MB for this example.
111 Create it by writing to the RAM disk device. (This step is not currently
112 required, but may be in the future.) It is wise to zero out the
113 area (esp. for disks) so that maximal compression is achieved for
114 the unused blocks of the image that you are about to create.
115116 dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
117118b) Make a filesystem on it. Say ext2fs for this example.
119120 mke2fs -vm0 /dev/ram0 2048
121122c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
123 and unmount it again.
124125d) Compress the contents of the RAM disk. The level of compression
126 will be approximately 50% of the space used by the files. Unused
127 space on the RAM disk will compress to almost nothing.
128129 dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
130131e) Put the kernel onto the floppy
132133 dd if=zImage of=/dev/fd0 bs=1k
134135f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
136 that is slightly larger than the kernel, so that you can put another
137 (possibly larger) kernel onto the same floppy later without overlapping
138 the RAM disk image. An offset of 400 kB for kernels about 350 kB in
139 size would be reasonable. Make sure offset+size of ram_image.gz is
140 not larger than the total space on your floppy (usually 1440 kB).
141142 dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
143144g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
145 For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
146 have 2^15 + 2^14 + 400 = 49552.
147148 rdev /dev/fd0 /dev/fd0
149 rdev -r /dev/fd0 49552
150151That is it. You now have your boot/root compressed RAM disk floppy. Some
152users may wish to combine steps (d) and (f) by using a pipe.
153154--------------------------------------------------------------------------
155 Paul Gortmaker 12/95
156157Changelog:
158----------
15916010-22-04 : Updated to reflect changes in command line options, remove
161 obsolete references, general cleanup.
162 James Nelson (james4765[AT]gmail[DOT]com)
16316416512-95 : Original Document